1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus for applying power from an electric motor to the steering system of a vehicle to lighten the manual steering effort of the driver.
2. Description of the Related Art
FIG. 6 of the accompanying drawings shows in block diagram a control unit 120 of a conventional electric power steering apparatus.
The control unit 120 generally includes a motor drive control section 130 and a motor drive section 140. In FIG. 6 reference character BAT denotes a battery power source, and reference numeral 125 denotes a motor current detecting unit or sensor for detecting a current flowing actually in an electric motor 110 and generating a motor current signal (hereinafter referred to as "motor current") IM corresponding to the detected motor current.
The motor drive control section 130 determines, on the basis of a steering torque signal T output from a steering torque sensor 112, a basic current value to be supplied to the electric motor 110 for enabling the electric motor 110 to produce a steering assist torque corresponding the steering torque, corrects the basic current value according to a vehicle velocity V detected by a vehicle velocity sensor 114, and outputs the corrected basic current value as a motor drive control signal (target current signal) IT. The motor drive control signal IT is supplied to the motor drive section 140.
The motor drive section 140 includes an offset calculating part or calculator 141, a PID (proportional plus integral plus derivative) control part or controller 142, a PWM (pulse-width modulation) signal generating part or generator 143, a gate drive circuit part or circuit 144, and a motor driving circuit 145 consisting of four power field-effect transistors (FET's) connecting in an H-type bridge.
The offset calculator 141 determines by calculation an offset between the motor drive control signal (target current signal) IT and the motor current IM detected by the motor current sensor 125 and outputs an offset signal 141a. The offset signal 141a is supplied to the PID controller 142.
The PID controller 142 performs PID (proportional plus integral plus derivative) control action about the offset signal 141a and generates and outputs a drive control signal 142a for controlling a current to be supplied to the electric motor 110 in such a way that the offset approaches zero. The drive control signal 142a is supplied to the PWM signal generator 143.
The PWM signal generator 143 generates a PWM signal 143a for PWM-driving the electric motor 110 on the basis of the drive control signal 142a and outputs the generated PWM signal 143a to the gate drive circuit 144. The gate drive circuit 144 drives the gates of the respective FET's on the basis of the PWM signal 143a.
The control unit 120 PWM-controls power supplied from the battery power source BAT to the electric motor 110 on the basis of the steering torque T detected by the steering torque sensor 112 and the vehicle velocity detected by the vehicle velocity sensor 114 and thereby controls the output power (steering assist torque) of the electric motor 110 which is to be applied to the steering system.
The force needed to steer front wheels (steerable wheels) of the vehicle, namely, the steering torque becomes a maximum when it is exerted while the vehicle is at rest, and this force becomes smaller as the vehicle velocity increases. In the conventional electric power steering apparatus, the ratio of manual steering torque (applied by the driver) and steering assist torque (applied by the electric motor 110), usually called "assist ratio", is varied according to the vehicle velocity to thereby improve the steering touch or feeling.
The steering torque to be exerted in conjunction with a start of the vehicle in the forward direction (hereinafter referred to as "forward start") is not the same as the steering torque to be exerted in conjunction with a start of the vehicle in the backward direction (hereinafter referred to as "backward start"). The steering torque is also varies with acceleration and deceleration of the vehicle. By virtue of the shift of a vehicle load caused due to acceleration and deceleration of the vehicle, the load or force acting on the front wheels increases when the vehicle makes a backward start or it is moving forward with decelerated velocity and decreases when the vehicle makes a forward start or it is moving forward with accelerated velocity. A change (increase or decrease) in the load on the front wheels directly causes a change in the so-called tire spring component (such as the spring characteristics, viscosity and friction of a mechanical system including tires that are variable with the ground-contact pressure or load). This phenomenon becomes remarkable as the weight of the vehicle or the level of the center of gravity of the vehicle increases.
Since the conventional electric power steering apparatus has no means for correcting the assist ratio in consideration of the forward start, backward start and acceleration and deceleration of the vehicle, a steering operation accompanying frequent forward and backward starts of the vehicle (which is due for parking or garaging, for example) cannot provide a steering assist well adaptable to the change of the tire spring component. Accordingly, due to an insufficient assist ratio, the force that must be exerted by the driver increases, the delay in assist response becomes large as the tire damping component increases, or the steering wheel is caused to oscillate. Thus, a good steering feel cannot be obtained.
Additionally, since the conventional electric power steering apparatus is generally tuned or adjusted in such a manner as to deal with only part of the forgoing problems arising from the forward start of the vehicle, the rest of the problems may still occur in conjunction with the backward start and acceleration and deceleration of the vehicle.